Encryption of digital data is a technical field which becomes important when transmitting and storing secret information or information which only shall be available to a user paying for the information. Thus, several methods for encrypting digital data are in frequent use. Such methods can also be applied also to digital image data. Examples of encryption methods are DES, triple DES and the public-key RSA method.
Digital images can be stored on servers and distributed over a telecommunication network as digital image data. Images can also be distributed using a physical storage medium such as a CD-ROM. Service providers need to establish access control that suits their business model. In this context it might be suitable to offer partial access to one set of users and full access to another set of users. Thus, some of the image data must be encrypted in order to prevent all users from having full access to all image data.
News photographs can e.g. be offered for sale on the Internet. The service provider wants to allow customers to download a version of the image with reduced quality for evaluation. Journals, that want to publish an image, pay for the service and are then allowed to download a full quality image.
However, such a service provider wants to minimize storage space and download bit rates. An image provider might alternatively want to distribute images on e.g. a CD-ROM. CD-ROMs are given away or sold for a low price. Customers can view the images at a reduced quality, but they must pay for viewing them at full quality. In the case the image provider wants to use the storage space on the CD-ROM as efficiently as possible.
It is also essential that customers always can access images using user friendly, standardised software. Image providers are reluctant to design and support special image viewers and customers don't want a proliferation of viewing tools.
Presently, image providers have to store two versions of the images stored. The full quality version is stored as an encrypted image file. This means that the image first is compressed and stored in a compressed file format such as JPEG or GIF. The compressed file is then encrypted using a suitable encryption tool and an encrypted image file is stored. The user must first decrypt this file and then access the resulting compressed image file using an image viewing tool. Reduced quality images are produced by processing the full quality images in an image editing program. They are stored as separate compressed image files.
The problems with this solution are that at least two different versions of the same image need to be stored, and that both versions must also be transmitted over the network in case of remote access in the case a customer first wants to see the free low resolution image before paying for the full resolution version.
This results in a significant disadvantage if the reduced version image contains a large fraction of the image information. Images that are offered for sale to journals would in particular be provided for preview at a fairly good quality since journal editors want to have a detailed understanding of the large content and accept only the highest quality for printing. The reduced quality image could require 10–50% of the storage space of the full quality image.
Furthermore, the emerging still image coding standard JPEG 2000, which is described in Charilaos Christopoulos (ed.), JPEG 2000 Verification Model Version 2.0, includes many new functionalities in comparison with other still image coding techniques. They include, in particular, methods for creating a wide range of progressive image formats. Each application domain can select a suitable progression mode. Individual objects within images can be accessed separately in the JPEG 2000 bitstream and progressive transmission can be applied also to objects. In JPEG 2000 there is also support for independently decodable coding units.